1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images in electrophotography, electrophotographic recording and electrophotographic printing, and to a fixer and an image forming apparatus using the toner.
2. Discussion of the Background
In electrophotographic image forming methods, a heating method upon application of pressure with a heat roller fixes a toner image on a recording medium by passing the recording medium through the surface of the heat roller, having releasability for the toner, while contacting the toner image thereon upon application of pressure. The method has quite good heat efficiency for fusion bonding the toner image on the recording medium because the toner image and the surface of the heat roller contact each other upon application of pressure, and can quickly fix the toner image thereon.
However, a part of the toner image adheres and transfers to the surface of the heat roller because the melted toner image is contacted thereto upon application of pressure, resulting in an offset problem wherein the adhered and transferred part of the toner image retransfers onto a following recording medium and contaminates the recording medium. The offset problem is largely affected by fixing speed and temperature.
Typically, when the fixing speed is low, the surface of the heat roller has comparatively a low temperature to make a heat quantity applied from the heat roller to the toner constant regardless of the fixing speed.
Particularly, in electrophotographic full-color image forming methods, plural toners are layered on a recording medium, and difference of temperatures between an uppermost layer thereof contacting a heat roller and a lowermost layer thereof contacting a recording medium become large when a fixing speed is high and the surface of the heat roller has a high temperature. The toner of the uppermost layer tends to have a hot offset problem. On the contrary, when the heat roller has a low temperature to prevent the hot offset problem, the toner of the lowermost layer is not fully melted, resulting in a cold offset problem wherein the toner is not fixed on the recording medium and adheres to the heat roller.
Recently, a toner having a wide range of fixable temperature, usable even when the fixing speed is high or low, and good offset resistance is required.
On the other hand, high-definition images having good thin line reproducibility are demanded. Therefore, a toner has a smaller particle diameter to increase image resolution and sharpness. However, the toner having a smaller particle diameter has, particularly when the fixing speed is high, low fixability in halftone images. This is because adhered quantity of the toner in halftone images is small, the toner transferred onto a concavity of a recording medium receives less heat quantity from a heat roller, and less pressure because a convexity thereof blocks a pressure to the concavity. In addition, the toner transferred onto the concavity of a halftone image on a recording medium has a thin layer and a pressure applied to a piece of the toner is higher than that of a solid image having a thick toner layer, resulting in occurrence of the offset problems and low-quality fixed images.
In order to make a toner have both fixability and offset resistance, a binder resin therein has been studied so far. Japanese Laid-Open Patent Publication No. 5-107803 discloses a molecular weight distribution of a resin having at least one peak in ranges of 103 to 7×104 and 105 to 2×106 respectively when measured by gel permeation chromatography (GPC).
Japanese Laid-Open Patent Publications Nos. 5-289399 and 5-313413 disclose a method of specifying a molecular weight of a vinyl copolymer and including a release agent such as polyethylene to make a toner have both fixability and offset resistance. Japanese Laid-Open Patent Publication No. 5-297630 discloses a method of improving low temperature fixability and hot offset resistance of a toner.
Japanese Laid-Open Patent Publications Nos. 5-053372, 6-027733, 6-075426 and 6-118702 disclose a method of widening a molecular weight of a binder resin, and balancing storage stability, fixability and hot offset resistance of the resultant toner.
Japanese Laid-Open Patent Publication No. 2002-372804 discloses a method of specifying a storage modulus of a toner to have good low-temperature fixability and hot offset resistance.
Conventional electrophotographic image forming apparatus includes a fixer wherein a pressure is pressed against a heating roller including a heat source, and a recording medium on which a toner image is transferred is passed therebetween to fix the
The fixer occasionally has an offset problem wherein the toner on the recording medium adheres to the heat roller. The offset toner also adheres to the pressure roller, and contaminates the recording medium when transferred therefrom. In order to prevent the offset, the surface of the heat roller in the conventional fixer has been fluorinated. However, it is difficult to completely prevent the offset due to environmental conditions and sorts of the recording medium.
Some conventional fixers have cleaners such as cleaning rollers removing a toner adhered to heat rollers and pressure rollers while contacting thereto. Such a cleaning member as is formed of a pure metal is pressed against the heat roller and pressure roller having improved surface releasability to remove the toner therefrom using a difference of the surface releasability therebetween.
Recently, image forming apparatuses stop supplying electricity to heat sources in standby states and do not supply electricity thereto until forming an image to heat the heat rollers to have a fixable temperature. Therefore, the temperature responsibility the heat roller needs to be improved, e.g., the heat roller has a thickness of 1 mm to shorten a warmup time to about 10 sec to have a fixable temperature.
In such an image forming apparatus, the heat roller has a small heat capacity and tends to have a nonuniform temperature distribution in the across-the-width direction due to a heat transfer to a recording medium when fixing or a member contacting thereto and a wind flow around the heat roller. In addition, the heat roller cannot have uniform temperature at all areas thereof in terms of space and cost.
When the heat roller has a nonuniform temperature distribution in the across-the-width direction, the fixability thereof becomes unstable and the offset tends to occur, and the heat roller has a shorter life due to heat deterioration. In Japanese Laid-Open Patent Publications Nos. 11-305577, 11-149180 and 2000-292981, an agglomerated polymerized toner adhered and accumulated on a cleaning member is melted again and transferred onto a recording medium to contaminate the recording medium. This is because the polymerized toner having a low storage modulus adheres to the cleaning member while a hard-to-melt pulverized toner having a high storage modulus adheres thereto.
A recording medium having a small size has more of this problem than a recording medium having a maximum passable size. This is because the recording medium having a small size has a small area contacting the heat roller, and a temperature of only the small area decreases and a temperature sensor turns on a heat source to needlessly increase a temperature of an area the recording medium does not pass through, resulting in melting of a toner on a cleaning member cleaning the area the recording medium does not pass through.
Japanese Laid-Open Patent Publication No. 9-325550 discloses a fixer preventing excessive increase of temperature of an area thereof a recording medium does not pass through by blowing a wind thereto to uniform a temperature distribution thereof in the across-the-width direction.
Japanese Laid-Open Patent Publication No. 9-325550 also discloses a fixer having a ventilator along a cleaning roller, circulating air therein with a rotation of the cleaning roller to prevent the cleaning roller from having an excessive high temperature.
Because of these reasons, a need exists for a fixer preventing a toner from melting out therefrom and contaminating images.